1994 GMC Yukon COOLANT LEAK SOMEWHERE AND SMELL and FLUCTU

MY VEHICLE IS RUNNING ALL OF A SUDDEN IN A PECULIAR MANNER AND I WANT TO CATCH THE PROBLEM EARLY. PLEASE HELP
FIRST OFF. MY OIL PRESSURE GAUGE IS LOWER THAN NORMAL WHILE STOPPED UNTIL I GET THE VEHICLE MOVING, IT ALSO WILL GO BEYOND THE HALFWAY MARK WHICH IS ABNORMAL. MY COOLANT GAUGE ALSO WILL FLUCTUATE COLD TO HOT AND IN TURN COLD AIR TEMPORARILY BLOWS ALSO. THE 'CHECK GAUGES' LIGHT WILL COME ON ALSO. ALL OF A SUDDEN MY COOLANT RESERVOIR IS EMPTY. NO IDEA WHERE IT IS LEAKING FROM, NO COOLANT LEAKING INTO THE OIL THAT I CAN SEE FROM THE DIPSTICK OR CAP. I CAN ALSO SMELL COOLANT IN THE VEHICLE BUT DON'T SEE ANY POOLING NOR DO I SEE A FILM ON THE WINDSHIELD. PLEASE HELP